The present invention is directed to methods and facilities for stabilizing and reducing solids content of excess activated sludge, foam, scum and the like that results from a biological wastewater treatment process or the like. More specifically, the invention is directed to a constant level anaerobic digester adapted to provide better handling of foam and gas by directing same into a central tower or elevated region that is equipped with gas venting and a weir overflow system to control sludge at a fixed level in the digester and in contact with most of the roof of the digester, while also providing piping and nozzle structure to control foam.
A functioning society produces an enormous amount of raw liquid sewage in the form of wastewater, which must be rendered innocuous prior to release into the environment. While some of this wastewater is treated chemically, the vast majority is treated using microorganisms that form a biomass that essentially eats, modifies or removes contaminants such as organic material, phosphorus and nitrates. Such processes especially convert organic materials into water, carbon dioxide and/or methane depending on the particular process being used. Such processes produce an excess of biomass or activated sludge because the microorganisms grow and multiply as they eat. Furthermore, stages of waste water treatment systems, such as primary clarifiers may produce scum which are removed from the clarified water. In most processes of this type, the excess activated sludge and scum are collected and must be treated in some manner. At some locations, these undesirable products may be sterilized, dried and used as fertilizer or transported to a landfill. Usually such processes are expensive and may result in substantial environmental problems and complaints from persons living in close proximity to the facility. Such process may also violate local or federal sludge disposal laws or guidelines.
Therefore, in some instances it is highly desirable to reduce the solids in the collected side streams by converting a portion of the solids to other substances such as gas that can be collected and beneficially used, especially methane gas, or that can be released, such as carbon dioxide. Such a process reduces the solids in the discharge components that must be later treated. A preferred system for such solids reduction is an anaerobic digester which utilizes microorganisms that live under anaerobic conditions and which modify much of the non-biomass organic material that remains in any side stream separated from the water treatment process and effectively eats or modifies other microorganisms in the sludge. Therefore, the streams are collected and placed into an anaerobic digester for stabilization and solids reduction. In such an anaerobic digester, the sludge is mixed and heated to maintain optimum conditions for certain organisms in the biomass to utilize other portions of the sludge as food so that the eaten organic material is stabilized.
The digestion process yields a number of by-products including methane gas released by biological activity of the organisms. Further, the mixing of the sludge along with the activity of the organisms has the potential to produce foam and scum, which rise to the surface of the digester contents creating a foam and scum layer. In many installations, scum from other areas of a facility may also be added to the contents of the digester. While some of the added scum is absorbed by the other contents or reduced in quantity by the microorganisms, some is not, thereby adding to the light material rising to the top of the digester. Controlling and removing the gas, especially methane gas, along with the scum and foam layer is essential in the proper operation of the digester. It is beneficial for the methane gas to be captured to prevent release into the environment and for preserving the methane to be used as a heat-producing gas at the facility. Therefore, the methane gas is collected from the top of the digester.
Most digesters of this type are relatively large and it is difficult to control the foam layer over a large surface. The collection of gas and foam can produce significant problems. In particular, the gas is normally withdrawn from digesters through ducting by downstream compression, which draws the methane and other gases out of the digester head space. If the foam gets out of control at the top of the digester, it can rise to the level of the gas take off and be drawn into the gas compressor system. Since such compressors are not designed for liquids or solids, foam drawn into such a compressor can seriously damage or destroy the machine. Therefore, it is desirable to control the foam layer by maintaining the top of the foam layer well below the gas take off during normal operation and by providing additional foam control apparatuses and methods of operation to control the foam layer, when needed.
Anaerobic digesters have long been employed for wastewater treatment processes. Designs for conventional shape digesters employ either fixed or floating dome covers that do not come into contact with the liquid. Fixed cover systems effectively control foam from escaping from the vessel; however, they do not limit foam generation within the vessel. Floating covers, typically used to provide gas storage within the vessel, also do not limit foam generation but have an added disadvantage of allowing foam to escape the vessel at the perimeter edge seals. Such covers are less effective in deterring foam formation, and do not promote re-entrainment of foam into the sludge to promote digestion while collecting methane gas in the uppermost part of the cover via a gas collection system. Such collection systems can lead to entrainment of foam into gas collection lines that are intended to capture only gas, thus potentially damaging compressors or other equipment.
Other prior art teaches an egg shaped vessel with a central dome that was developed to promote full mixing of the contents. However, such designs are relatively expensive and the design itself does not effectively resolve the problems of handling foam and collecting gas without foam getting into gas lines.